The invention concerns a nuclear magnetic resonance (NMR) magic angle spinning (MAS) probe head for measuring a measuring substance in an MAS rotor, with a bottom box and a tube that is mounted to the bottom box and projects therefrom, wherein an MAS stator is disposed within the tube in the area of the end of the tube facing away from the bottom box for receiving an MAS rotor, and with a pneumatic sample changing system for supplying and removing an MAS rotor to/from the MAS stator.
An NMR-MAS probe head of this type is disclosed e.g. in the technical poster “Development of a CryoMAS™ HR-MAS-MAG NMR Probe for High-field WB Magnets”, Sid Shevgoor et al., Doty Scientific, Columbia, S.C., USA presented at ENC 2005, Providence, R.I., USA, accessible on the internet on Oct. 9, 2008 at www.dotynmr.com/PDF/CryoMAS_ENC05.pdf.
Nuclear magnetic resonance (NMR) spectroscopy is a method of instrumental analysis for determining, in particular, the chemical composition of measuring samples. Radio frequency (RF) pulses are thereby irradiated into the measuring sample that is located in a strong static magnetic field, and the electromagnetic response of the sample is measured.
An NMR sample is conventionally rotated while tilted at the so-called “magic angle” of approximately 54.74° with respect to the static magnetic field (“MAS”=Magic Angle Spinning) during spectroscopic measurement in order to reduce line broadenings due to anisotropic interactions. Towards this end, the sample is filled into an MAS rotor. MAS rotors are small cylindrical tubes that are open on one side and are closed by a cap, wherein the cap has wing elements (“small fan wheels”). The MAS rotor is disposed in an MAS stator and the MAS rotor is rotationally driven by gas pressure via the wing elements. The MAS rotor and the MAS stator as a whole are called an MAS turbine.
During the NMR measurement, the MAS turbine is disposed in an NMR-MAS probe head. The probe head has a cylindrical shielding tube (also briefly called “tube”) and a bottom box. Radio frequency (RF) electronic components, in particular RF resonator coils, and the MAS turbine are housed in the tube, wherein the MAS turbine is disposed in the area of the end of the tube facing away from the bottom box. The shielding tube of the probe head is typically introduced from below into the vertical room temperature bore of a superconducting magnet, is positioned and held by hooks, supports, screws or the like. The MAS turbine is then located exactly in the magnetic center of the magnet.
With simple probe heads, the probe head must be removed from the magnet, i.e. the probe head must be removed from the room temperature bore, for changing an NMR probe or an MAS rotor filled with measuring substance. Towards this end, the user kneels below the magnet, releases the holders and cable connections and receives the probe head when it slides out of the magnet. Removal of the probe head or also reinsertion of the probe head into the magnet may require a considerable amount of force due to eddy currents induced in the metal parts of the probe head, in particular in the shielding tube, and the net weight of the probe head. The manufacturers of probe heads stipulate in their instructions that a probe head be removed by two persons together for reasons of safety. The rotor can then be manually changed on the removed probe head. A rotor change, i.a. new positioning of the probe head in the magnet, requires in most cases renewed shimming such that this overall procedure is very complex.
According to document DE 38 18 039 A1, a rotatable sample magazine is provided in the direct vicinity of the MAS stator on the probe head such that the sample in the MAS stator can be changed several times through application of gas pressure without removing the probe head or the probe magazine from the inside of the magnet.
The technical poster of Shevgoor et al. discloses the use of a lifting system for MAS rotors. A transport conduit is connected to the end of the tube of a probe head facing away from the bottom box. The transport conduit extends through the room temperature bore of a magnet in an upward direction and out of the magnet. An MAS rotor can be supplied by gas pressure through the transport conduit into the MAS stator of the probe head mounted in the magnet, and an MAS rotor can also be removed from the MAS stator in an upward direction out of the probe head.
Due to the fact that the transport conduit is guided through the room temperature bore, the room temperature bore is obstructed, in particular, also above the magnet, thereby increasing the instrumental expense. The transport conduit also aggravates initial mounting of the probe head. The transport conduit must also be guided through the wall of the shielding tube to the MAS stator, thereby aggravating RF shielding of the sample during the NMR measurement.
Moreover, many cases require measurement of the sample under defined extreme temperature conditions, in particular at cryogenic temperatures (−196° C. or less). The inside of the shielding tube is thereby temperature-controlled or cooled. In this case, passage of the transport conduit through the tube at the end remote from the bottom box represents a heat conducting bridge that aggravates keeping of the defined extreme temperature conditions.
It is the underlying purpose of the invention to provide an NMR-MAS probe head which enables fast change between different MAS rotors, and facilitates RF shielding and keeping of defined extreme temperature conditions.